The forming of a solution of a material which is quite viscous by dissolving a solid particulate powder of the material in a solvent, and then efficiently stirring the resulting mixture until the power is dissolved, during which time the solution becomes highly viscous, has been rather difficult to accomplish. Many commercially available impellers for carrying out such stirring have been evaluated in a mixing vessel, and have failed because of one or more of the following reasons:
1. All of the flow of liquid from the impeller is radial, which generates discrete liquid flow zones in the vessel resulting in minimal intermixing in these zones. PA1 2. Production of stagnant layers and zones in the liquid as a result of the particular design of the blades of the impeller. PA1 3. Poor shear action of the impeller on the liquid, which results in ineffective dissolution of the particulate material in the solvent. PA1 4. The action of the impeller in pumping the liquid around in the vessel is too low to effectively mix two liquid reactive components which produce a viscous solution in a time period within which the components react. PA1 5. Too much swirling of the liquid by the impeller, with the subsequent production of vortexes and entrapment of gas in the solution.
In order to minimize the swirling of the viscous liquid being mixed, it has been proposed to provide in the mixing vessel baffles consisting of flat blades. When such baffles are evaluated, however, although the vortexing and swirling of the liquid is reduced, stagnant zones are formed directly on the back side of such baffles as a result of the highly viscous nature of the liquid and the resulting flow pattern.